Generally, in order to increase the recording density of optical disk devices, it is necessary either to shorten the wavelength of the light used to record and reproduce optical disks serving as information recording media or to increase the numerical aperture (NA) of the objective lens that focuses the light onto the optical disk.
If the numerical aperture of the objective lens is increased, then a large spherical aberration is generated because of variations in the thickness of the substrate of the optical disk, and in the case of multi-layer structured optical disks, changes in the thickness of the substrate when switching between information recording and reproduction surfaces. If the numerical aperture of the objective lens further is enlarged, then a large coma aberration occurs if a relative inclination (referred to below as “tilt”) is generated between the optical axis of the objective lens and the surface of the optical disk.
Due to these spherical and coma aberrations, light spots focused on the optical disk deteriorate, and information recording and reproduction capability is lost. Thus, for optical disk devices in which the recording density is high, there is a need to detect these aberrations and correct them.
For spherical aberration detection means in conventional optical disk devices, the spherical aberration detection means described in JP 2000-171346A is known.
FIG. 20 shows a structural overview of a conventional optical head 90 that is capable of detecting spherical aberration.
In FIG. 20, numeral 101 denotes a semiconductor laser, numeral 104 denotes a collimator lens, numeral 105 denotes an objective lens, numeral 106 denotes an optical disk, numeral 107 denotes a hologram, and numeral 108 denotes a photodetector.
A laser light emitted from the semiconductor laser 101 is converted to substantially parallel light by the collimator lens 104, and is focused by the objective lens 105 through a substrate of the optical disk 106 onto a recording and reproduction information surface. The laser light reflected by the recording and reproduction information surface of the optical disk 106 again passes through the substrate, passes through the objective lens 105 and the collimator lens 104, passes through the hologram 107 and is diffracted to be incident on the signal detecting photodetector 108.
The hologram 107 has a pattern as shown in FIG. 21.
The hologram 107 contains three regions: a first region “a” that is bounded by a straight line L that is perpendicular to the optical axis, and a first semicircle E1 that is centered on the optical axis; a second region “b” that is bounded by the first semicircle E1, a second semicircle E2 that has a radius larger than the semicircle E1 and is positioned on the same side of the straight line L as the semicircle E1, and the straight line L; and a third region “c” that is bounded by a third semicircle E3 that is on the opposite side of the straight line L from the first semicircle E1 and the second semicircle E2, and the straight line L.
The regions “a”, “b” and “c” of the hologram 107 are configured such that the focal spots of the light passing through the regions “a”, “b” and “c” from the optical disk 106 side, corresponding to the regions “a”, “b” and “c” are focused separately on the photodetector 108. That is to say, the light passing through the three regions “a”, “b” and “c” of the hologram 107 from the optical disk 106 side is formed as focal spots in three locations on the photodetector 108.
As shown in FIG. 22A to FIG. 22C, the photodetector 108 is configured by five light receiving regions 108a to 108e. The light flux from the first region “a” of the hologram 107, of the light fluxes of the laser light reflected by the optical disk 106, is formed as a focal spot P1 on the borderline of the light receiving regions 108a and 108b, the light flux from the second region “b” of the hologram 107 is formed as a focal spot P2 on the borderline of the light receiving regions 108c and 108d, and the light flux from the third region “c” is formed as a focal spot P3 in the light receiving region 108e. 
Thus, when an information signal (reproduction signal) RF recorded on the optical disk 106 is expressed using output electric signals from the light receiving regions 108a to 108e, it is given by:
Reproduction signal RF=signal obtained by the light receiving region 108a+signal obtained by the light receiving region 108b+signal obtained by the light receiving region 108c+signal obtained by the light receiving region 108d+signal obtained by the light receiving region 108e. 
When the substrate of the optical disk 106 is suitable and there is no generation of spherical aberration and when the focal point is correctly formed on the optical disk 106, that is to say, when it is focused, the shape of the focal spots P1 to P3 formed on the light receiving regions 108a to 108e are spots of substantially the same size, as shown in FIG. 22B.
Thus, the focal spot P1 of the light flux diffracted at the hologram 107 is formed such that the irradiated areas of the light receiving regions 108a and 108b are equal. That is to say, this indicates that the values of the electric signal obtained from the light receiving region 108a and the electric signal obtained from the light receiving region 108b are equal. In a similar manner, the focal spot P2 is formed such that the irradiated areas of the light receiving regions 108c and 108d are equal.
Generally, if the thickness of the substrate of the optical disk 106 is not suitable, then spherical aberration occurs in focusing optical systems having the above-noted configuration.
FIG. 23 shows the state of the light rays when spherical aberration occurs. When spherical aberration occurs, there is generation of a shift in the focal position that depends on the distance of the light ray from an optical axis “o”. That is to say, when the light ray “b” is focused on a surface “F”, the light ray “a”, which is further from the optical axis “o” than the light ray “b”, is focused in front of the surface “F”, and the light ray “c”, which is further closer to the optical axis “o” than the light ray “b”, is focused behind the surface “F”.
That is to say, by detecting the state of the focus in two regions whose distance from the optical axis “o” is different from each other, it is possible to know the spherical aberration situation.
If spherical aberration has occurred in the focusing optical system, then even if the system is focused, that is to say, even if the difference in the electric signal between the light receiving region 108a and the light receiving region 108b is 0, then the difference in the electric signal between the light receiving region 108c and the light receiving region 108d is not 0, but takes on a positive or negative value. Thus, this indicates that a positive or negative spherical aberration has occurred.
If a positive or negative spherical aberration occurs in the above-noted focusing optical system, then assuming, for example, that a positive spherical aberration occurs, since the focal position of the focal spot P2 of the light receiving regions 108c and 108d, which is a light flux of the second light ray “b”, which is a further distance from the optical axis, is in front of the light receiving surface of the photodetector 108, the focal spot P2 is enlarged in a half donut-shape over the light receiving region 108d as shown in FIG. 21A. Conversely, when a negative spherical aberration occurs, since the focal position of the focal spot P2 of the light receiving region 108c and the light receiving region 108d is behind the light receiving surface of the photodetector 108, the focal spot P2 is enlarged in a half donut-shape over the light receiving region 108c as shown in FIG. 22C.
Consequently, a spherical aberration signal SAE, which is a signal indicating that spherical aberration has occurred in the focusing optical system, is as given below.
Spherical aberration signal SAE=signal obtained by the light receiving region 108c−signal obtained by the light receiving region 108d−K×(signal obtained by the light receiving region 108a−signal obtained by the light receiving region 108b), where K is a constant.
It should be noted that the means described in JP H8-212611A is known as aberration correction means for correcting spherical aberration. If there is a change in the substrate thickness of optical disks, then liquid crystal elements are controlled to correct the aberration in accordance with a spherical aberration detection signal.
Liquid crystal elements are elements in which a liquid crystal is sealed in a section that is sandwiched between two glass substrates. When the part through which laser light passes is divided into a plurality of regions and an independent voltage is applied to each region, it is possible to change the refractive index of the corresponding parts. It is possible to alter the phase of the wavefronts by utilizing these changes in the refractive index. Since the phase of the laser light changes sectionally when the laser light contains aberrations, the aberrations can be corrected by activating the liquid crystal elements so as to complement the altered phases. When a voltage is applied in accordance with the degree of aberration, it is possible to correct the aberrations with greater accuracy. If spherical aberration has occurred, then the phase of the liquid crystal elements is controlled so as to minimize wavefront aberration.
The means described in International Application PCT/JP01/05366 is known as conventional tilt detecting means for optical disk devices.
FIG. 24 shows a structural overview of a conventional optical head 80 which is capable of tilt detection.
In FIG. 24, numeral 201 denotes a semiconductor laser, numeral 202 denotes a beam splitter, numeral 204 denotes a collimator lens, numeral 205 denotes an objective lens, numeral 206 denotes an optical disk, numeral 207 denotes a relay lens and numeral 208 denotes a photodetector.
The laser light emitted from the semiconductor laser 201 passes through the beam splitter 202, is converted to substantially parallel light by the collimator lens 204, and passes through the substrate to be focused on the recording and reproduction information surface of the optical disk 206 by the objective lens 205.
The laser light reflected by the recording and reproduction information surface of the optical disk 206 again passes through the substrate, passes through the objective lens 205 and the collimator lens 204, is reflected by the beam splitter 202 and is guided to the signal detecting photodetector 208 by the relay lens 207.
The light fluxes incident on the photodetector 208 as shown in FIG. 25A are divided into six parts and are received by the light receiving regions 208a to 208f. A first tracking error signal TE1 is detected using the signals received by the light receiving regions 208e and 208f, and a second tracking error signal TE2 is detected using the signals received by the light receiving region 208a to the light receiving region 208d. 
Since the tracking error signals are push pull signals, the tracking error signals TE1 and TE2 are expressed by the following formula.
Tracking error signal TE1=signal received by the light receiving region 208e−signal received by the light receiving region 208f. 
Tracking error signal TE2=(signal received by the light receiving region 208a+signal received by the light receiving region 208b)−(signal received by the light receiving region 208c+signal received by the light receiving region 208d).
The inclination (tilt) of the optical disk can be detected by comparing the phases of the first tracking error signal TE1 and the second tracking error signal TE2.
FIG. 25B is a diagram in which the scope of the light receiving regions is superimposed on a distribution of the light intensity of the detected light fluxes when the optical disk is tilted in the radial direction. Although there is an asymmetry in the intensity distribution of the light fluxes, in accordance with the tilt of the optical disk, a large portion of that asymmetry occurs on the light receiving regions 208e and 208f, as shown in FIG. 25B. Thus, the degree of influence of the tilt of the optical disk differs between the first tracking error signal TE1 and the second tracking error signal TE2.
When the optical disk is not tilted, the phase of the two tracking error signals TE1 and TE2 coincide, but when the optical disk tilts, a phase shift is created between the two-tracking error signals-TE1 and T-E2. Since the degree of influence of the tilt of the optical disk differs for each signal, it is possible to detect the tilt of the optical disk by comparing the phases of the first tracking error signal TE1 and the second tracking error signal TE2.
It should be noted that even when a conventional push pull signal, that is to say, a push pull signal TE3 obtained by the calculation,
push pull signal TE3=(signal received by the light receiving region 208a+signal received by the light receiving region 208b+signal received by the light receiving region 208e)−(signal received by the light receiving region 208c+signal received by the light receiving region 208d+signal received by the light receiving region 208f),
is used as the first tracking error signal TE1, it is possible to detect the tilt of the optical disk. This is because, since the push pull signal TE3 corresponds to a signal that is the sum of the tracking error signals TE1 and TE2, a sufficiently detectable phase difference is generated between the first and second tracking error signals TE1 and TE2.
However, in a conventional configuration, because the spherical aberration detection means and the tilt detecting means are configured independently for detecting both aberrations, there is the problem of mutual interference between the pattern of the hologram and the pattern of the photodetector. Alternatively, because the number of divisions of the light fluxes increases, the electric signal detected by the light receiving regions decreases, there is the problem that the S/N ratio necessary for detecting aberration cannot be obtained.
It is an object of the present invention to realize simultaneously spherical aberration detection and tilt detection using the configuration of a simple hologram and photodetector, and also to provide an optical head of a simple configuration that includes a tracking error signal detection and focus error signal detection capable of correcting offset when the objective lens is moved.